Toll-like receptor is expressed by various immune cells and recognizes high reserved structural motifs: Pathogen Associated Molecular Pattern (PAMP) expressed by microorganism pathogens or Damage Associated Molecular Patterns (DAMP) released by dead cells. PAMP or DAMP stimulates Toll-like receptor to trigger signal cascade which induces the activations of transcriptional factors like AP-1, NF-κB and interferon regulators (pulse response function). It results in various cell responses, including productions of interferons, proinflammatory cytokines and effector cytokines, whereby immune response is produced.
By far, 13 types of Toll-like receptors have been discovered in mammal. Toll-like receptors 1, 2, 4, 5 and 6 are mainly expressed on the cell surface while Toll-like receptors 3, 7, 8 and 9 are expressed in the endosome. Different Toll-like receptors recognize ligands derived from different pathogens. Toll-like receptor 7 (TLR7) is expressed and ligand recognized by plasmaeytoid dendritic cells (pDC) to induce the secretion of interferon α (IFN-α). Toll-like receptor 7 (TLR7) and Toll-like receptor 8 (TLR8) are highly homologous and therefore the ligand of TLR7 in many cases is also that of TLR8. TLR8 stimulation mainly induces the productions of cytokines like tumor necrosis factor α (TNF-α) and chemoattractant. Interferon α is one of the main medicines for treating chronic hepatitis B or hepatitis C while TNF-α is a proinflammatory cytokine, of which the over secretion will result severe side effects.
There have been reported several TLR7 agonists, like Imiquimod (British Journal of Dermatology 2003; 149 (Suppl. 66): 5-8), Resiquimod (Antiviral Research 64 (2004) 79-83), GS-9620 (Gastroenterology (2013), 144(7), 1508-1517). Nevertheless, it is desirable to have novel TLR7 agonists with better selectivity, activity and safety.